The invention herein relates to friction materials suitable for railroad service.
Over the years a number of different types of composition railroad brake shoes have been described in the prior art. These have normally been composed of a rubber and/or resin matrix heavily reinforced with asbestos fiber and containing lead in the form of elemental lead and/or lead oxides as critical components. Typical compositions have been described in U.S. Pat. Nos. 2,686,140; 2,861,964; 3,152,099; 3,390,113; 3,832,325 and 3,959,194. Many of the materials described in the aforementioned patents have enjoyed widespread commercial success as railroad brake shoe compositions under the trademark COBRA. However, environmental and health concerns have led manufacturers to seek friction material compositions which contain neither asbestos fiber nor lead metal or its compounds. Recently, a composition containing little or no lead was disclosed. This was found to be quite advantageous in reducing wheel wear, particularly where softer steels are used. See the aforesaid U.S. Pat. No. 3,959,194. Efforts to eliminate asbestos fiber from the brake shoe compositions have been beset with major difficulties, primarily because the asbestos fiber contributed a unique combination of reinforcement and thermal properties to the compositions. During service application brake shoes undergo significant physical stresses and also experience substantial temperature increases, often reaching peak temperatures in excess of 800.degree. F. Many fibrous materials which might have been expected to be suitable as substitutes for asbestos fiber, such as glass fiber and organic fiber, have been found to be unable to adequately withstand the thermal and physical forces involved in the heavy duty service.
Recently a friction material manufacturer announced the replacement of asbestos fiber in railroad brake shoes with an otherwise undefined "all-metal alloy." It is not disclosed whether such material is in fibrous or other form.
Consequently it would be desirable to have a railroad friction material which contains neither lead nor asbestos.
Further, in the past it has been common practice to provide a different type of friction material composition for different types of railroad service. Conventional AAR (Association of American Railroads) service for standard freight cars constitutes the large majority of the uses of friction materials and therefore many compositions were directed to such end use. However, other uses also require significant quantites of friction materials, such as the heavy duty service (primarily brake shoes for locomotives) and disc brakes (which can be considered to be relatively low pressure, high friction service). The friction materials which have been provided for these other services have been entirely different types of compositions from those designed for the AAR service. Thus friction material manufacturers have had to develop product lines of entirely different types of materials with the attendant high costs of development and fabrication, which added costs were necessarily reflected in the equipment purchase costs of the railroad users.
Consequently, it would be very advantageous to have a basic friction material composition which in different embodiments could be used as an AAR service friction material, a heavy duty service friction material, or a disc brake friction material.